Activation and reaction kinetics of the dimethylamine/coenzyme M methyltransfer in Methanosarcina barkeri strain Fusaro

Dimethylamine/5-hydroxybenzimidazolylcobamide methyltransferase (DMA-MT) from Methanosarcina barkeri Fusaro catalyzes (Vmax = 4700 nmol x min(-1) x mg(-1) protein; k(cat) = 7.8 s(-1)) the transfer of a methyl group from dimethylamine (apparent Km = 0.45 mM) to its corrinoid prosthetic group to yield monomethylamine (MMA) and the methylated enzyme. The product, MMA, is a competitive inhibitor of the reaction (apparent Ki = 5.5 mM). The methyl group bound to the corrinoid prosthetic group of DMA-MT is subsequently transferred to coenzyme M in a reaction mediated by methylcobalamin/coenzyme M methyltransferase isoenzyme II [MT2(II)], which binds with high affinity to DMA-MT (apparent Km = 0.22 microM). As isolated, DMA-MT is inactive, but it can enzymically be reactivated by methyltransferase activating protein (MAP), ATP, and hydrogenase. Apart from the established role in corrinoid activation, ATP was found to act as a powerful allosteric effector on the methyltransferase reaction. The results of kinetic studies, supported by the resolution of as-yet partially purified auxiliary protein fractions, demonstrate that DMA-MT, MT2(II), MAP, and hydrogenase are the only enzymic components involved in the dimethylamine/coenzyme M methyltransfer in M. barkeri Fusaro.

Isolation and characterization of Methanobacterium thermoautotrophicum DeltaH mutants unable to grow under hydrogen-deprived conditions

By using random mutagenesis and enrichment by chemostat culturing, we have developed mutants of Methanobacterium thermoautotrophicum that were unable to grow under hydrogen-deprived conditions. Physiological characterization showed that these mutants had poorer growth rates and growth yields than the wild-type strain. The mRNA levels of several key enzymes were lower than those in the wild-type strain. A fed-batch study showed that the expression levels were related to the hydrogen supply. In one mutant strain, expression of both methyl coenzyme M reductase isoenzyme I and coenzyme F420-dependent 5,10-methylenetetrahydromethanopterin dehydrogenase was impaired. The strain was also unable to form factor F390, lending support to the hypothesis that the factor functions in regulation of methanogenesis in response to changes in the availability of hydrogen.

Purification and characterization of dimethylamine:5-hydroxybenzimidazolyl-cobamide methyltransferase from Methanosarcina barkeri Fusaro

Dimethylamine:5-hydroxybenzimidazolylcobamide methyltransferase (DMA-MT) was purified from cells of Methanosarcina barkeri Fusaro grown on trimethylamine. In the presence of methylcobalamine:coenzyme M methyltransferase isoenzyme II [MT2(II)] the enzyme quite specifically catalyzed the stoichiometric conversion of dimethylamine (apparent Km = 0.45 mM) and 2-mercaptoethane-sulfonate (coenzyme M) to monomethylamine and methyl-coenzyme M. Monomethylamine was a competitive inhibitor of the reaction (Ki = 4.5 mM). The apparent molecular mass of DMA-MT was 100 kDa and the enzyme was found to be a dimer, composed of identical 50-kDa subunits. A corrinoid content of 0.9 +/- 0.1 mol B12/mol holoenzyme was calculated from HPLC analysis. The as-isolated methyltransferase was inactive, but it could be reductively reactivated. Activation required the presence of methyltransferase-activating protein, ATP and dimethylamine. Incubation with these compounds resulted in the methylation of the corrinoid prosthetic group.

Cellular levels of factor 390 and methanogenic enzymes during growth of Methanobacterium thermoautotrophicum deltaH

Methanobacterium thermoautotrophicum deltaH was grown in a fed-batch fermentor and in a chemostat under a variety of 80% hydrogen-20% CO2 gassing regimes. During growth or after the establishment of steady-state conditions, the cells were analyzed for the content of adenylylated coenzyme F420 (factor F390-A) and other methanogenic cofactors. In addition, cells collected from the chemostat were measured for methyl coenzyme M reductase isoenzyme (MCR I and MCR II) content as well as for specific activities of coenzyme F420-dependent and H2-dependent methylenetetrahydromethanopterin dehydrogenase (F420-MDH and H2-MDH, respectively), total (viologen-reducing) and coenzyme F420-reducing hydrogenase (FRH), factor F390 synthetase, and factor F390 hydrolase. The experiments were performed to investigate how the intracellular F390 concentrations changed with the growth conditions used and how the variations were related to changes in levels of enzymes that are known to be differentially expressed. The levels of factor F390 varied in a way that is consistently understood from the biochemical mechanisms underlying its synthesis and degradation. Moreover, a remarkable correlation was observed between expression levels of MCR I and II, F420-MDH, and H2-MDH and the cellular contents of the factor. These results suggest that factor F390 is a reporter compound for hydrogen limitation and may act as a response regulator of methanogenic metabolism.

Medium-reductant directed expression of methyl coenzyme M reductase isoenzymes in Methanobacterium thermoautotrophicum (strain deltaH)

Methanobacterium thermoautotrophicum was grown in a chemostat under various controlled conditions in the presence of either sodium sulfide or sodium thiosulfate. After establishment of the steady state, cells were taken and examined for expression of the mRNA transcripts coding for the different forms of methyl coenzyme M reductase (MCR) and methylene tetrahydomethanopterin dehydrogenase (MDH). MCR isoenzyme II expression varied most markedly. Expression was found not only to depend on known parameters temperature, pH and gassing rate, but also on the medium composition, especially the reductant present.

Involvement of methyltransferase-activating protein and methyltransferase 2 isoenzyme II in methylamine:coenzyme M methyltransferase reactions in Methanosarcina barkeri Fusaro

The enzyme systems involved in the methyl group transfer from methanol and from tri- and dimethylamine to 2-mercaptoethanesulfonic acid (coenzyme M) were resolved from cell extracts of Methanosarcina barkeri Fusaro grown on methanol and trimethylamine, respectively. Resolution was accomplished by ammonium sulfate fractionation, anion-exchange chromatography, and fast protein liquid chromatography. The methyl group transfer reactions from tri- and dimethylamine, as well as the monomethylamine:coenzyme M methyltransferase reaction, were strictly dependent on catalytic amounts of ATP and on a protein present in the 65% ammonium sulfate supernatant. The latter could be replaced by methyltransferase-activating protein isolated from methanol-grown cells of the organism. In addition, the tri- and dimethylamine:coenzyme M methyltransferase reactions required the presence of a methylcobalamin:coenzyme M methyltransferase (MT2), which is different from the analogous enzyme from methanol-grown M. barkeri. In this work, it is shown that the various methylamine:coenzyme M methyltransfer steps proceed in a fashion which is mechanistically similar to the methanol:coenzyme M methyl transfer, yet with the participation of specific corrinoid enzymes and a specific MT2 isoenzyme.

Activation mechanism of methanol:5-hydroxybenzimidazolylcobamide methyltransferase from Methanosarcina barkeri

Methanol:5-hydroxybenzimidazolylcobamide methyltransferase (MT1) is the first of two enzymes involved in the transmethylation reaction from methanol to 2-mercaptoethanesulfonic acid in Methanosarcina barkeri. MT1 only binds the methyl group of methanol when the cobalt atom of its corrinoid prosthetic groups is present in the highly reduced Co(I) state. Formation of this redox state requires H2, hydrogenase, methyltransferase activation protein, and ATP. Optical and electron paramagnetic resonance spectroscopy studies were employed to determine the oxidation states and coordinating ligands of the corrinoids of MT1 during the activation process. Purified MT1 contained 1.7 corrinoids per enzyme with cobalt in the fully oxidized Co(III) state. Water and N-3 of the 5-hydroxybenzimidazolyl base served as the upper and lower ligands, respectively. Reduction to the Co(II) level was accomplished by H2 and hydrogenase. The cob(II)amide of MT1 had the base coordinated at this stage. Subsequent addition of methyltransferase activation protein and ATP resulted in the formation of base-uncoordinated Co(II) MT1. The activation mechanism is discussed within the context of a proposed model and compared to those described for other corrinoid-containing methyl group transferring proteins.

Purification and properties of an enzyme involved in the ATP-dependent activation of the methanol:2-mercaptoethanesulfonic acid methyltransferase reaction in Methanosarcina barkeri

In Methanosarcina barkeri the transfer of the methyl group from methanol to 2-mercaptoethanesulfonic acid is catalyzed by the concerted action of two methyltransferases. The first one is the corrinoid-containing methanol:5-hydroxybenzimidazolylcobamide methyltransferase (MT1), which binds the methyl group of methanol to its corrinoid prosthetic group. MT1 is only catalytically active when the cobalt atom of the corrinoid is present in the highly reduced Co(I) state. In the course of its purification and even during catalysis, MT1 becomes oxidatively inactivated. The enzyme, however, may be reductively reactivated by a suitable reducing system (hydrogen and hydrogenase), ATP, and an enzyme called methyltransferase activation protein (MAP). In order to elucidate its role in the reactivation process, MAP was purified to apparent homogeneity. The protein had an Mr = 60,000. Preincubation of the enzymic components involved with 8-azido-ATP or with ATP demonstrated MAP to be the primary site of action of ATP. In agreement herewith, the protein was autophosphorylated by [gamma-32P]ATP in a 1:1 stoichiometry. Phosphorylated MAP substituted for ATP in the activation of MT1, and the addition of increasing amounts of MAP phosphate resulted in a corresponding increase of active MT1. However, in the presence of limiting amounts of MAP, maximal activation of MT1 could be achieved during a lag phase provided ATP was present, indicating that MAP acts as a catalyst. This paper is the first to report on the presence, isolation, and function of a phosphorylated protein in a methanogenic archaeon.

Metabolic regulation in methanogenic archaea during growth on hydrogen and CO2

Methanogenic Archaea represent a unique group of micro-organisms in their ability to derive their energy for growth from the conversion of their substrates to methane. The common substrates are hydrogen and CO2. The energy obtained in the latter conversion is highly dependent on the hydrogen concentration which may dramatically vary in their natural habitats and under laboratory conditions. In this review the bio-energetic consequences of the variations in hydrogen supply will be investigated. It will be described how the organisms seem to be equipped as to their methanogenic apparatus to cope with extremes in hydrogen availability and how they could respond to hydrogen changes by the regulation of their metabolism.

Coenzyme F390 synthetase from Methanobacterium thermoautotrophicum Marburg belongs to the superfamily of adenylate-forming enzymes

Depending on the reduction-oxidation state of the cell, some methanogenic bacteria synthesize or hydrolyze 8-hydroxyadenylylated coenzyme F420 (coenzyme F390). These two reactions are catalyzed by coenzyme F390 synthetase and hydrolase, respectively. To gain more insight into the mechanism of the former reaction, coenzyme F390 synthetase from Methanobacterium thermoautotrophicum Marburg was purified 89-fold from cell extract to a specific activity of 0.75 mumol.min-1.mg of protein-1. The monomeric enzyme consisted of a polypeptide with an apparent molecular mass of 41 kDa as determined by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. ftsA, the gene encoding coenzyme F390 synthetase, was cloned and sequenced. It encoded a protein of 377 amino acids with a predicted M(r) of 43,280. FtsA was found to be similar to domains found in the superfamily of peptide synthetases and adenylate-forming enzymes. FtsA was most similar to gramicidin S synthetase II (67% similarity in a 227-amino-acid region) and sigma-(L-alpha-aminoadipyl)-L-cysteine-D-valine synthetase (57% similarity in a 193-amino-acid region). Coenzyme F390 synthetase, however, holds an exceptional position in the superfamily of adenylate-forming enzymes in that it does not activate a carboxyl group of an amino or hydroxy acid but an aromatic hydroxyl group of coenzyme F420.

Purification and properties of coenzyme F390 hydrolase from Methanobacterium thermoautotrophicum (strain Marburg)

8-Hydroxyadenylylated coenzyme F420 (coenzyme F390-A) is formed in methanogenic bacteria upon oxidative stress. After reinstatement of anaerobic conditions, coenzyme F390 is degraded into coenzyme F420 and AMP. The enzyme catalyzing the latter reaction, coenzyme F390 hydrolase, was purified to homogeneity from Methanobacterium thermoautotrophicum strain Marburg 355-fold to a specific activity of 12.1 mumol.min-1.mg protein-1. The enzyme consisted of one polypeptide of approximately 27 kDa. Coenzyme F390 hydrolase displayed an apparent Km for coenzyme F390 of 40 microM. The enzyme required the presence of a reducing agent like dithiothreitol to become active. Activity could be manipulated by applying various ratios of reduced and oxidized dithiothreitol. Activation proceeded by a two-electron reduction, which indicates that one S-S bridge is involved the activation/inactivation of the enzyme. Dithiothreitol could be replaced by the methanogenic C1-carrier 2-mercaptoethanesulfonate (H-S-CoM), but not by N7-mercaptoheptanoyl-L-threonine phosphate (H-S-HTP) or other naturally occurring thiol-containing compounds. The addition of the heterodisulfide of H-S-CoM and H-S-HTP (CoM-S-S-HTP) diminished the stimulatory effect of H-S-CoM.

The electrochemistry of 5-hydroxybenzimidazolylcobamide

Methanogenic archaea typically contain 5-hydroxybenzimidazolylcobamide (cba-HBI) as the prosthetic group of a number of methyltransferases involved in their central metabolic pathways. In this paper the (acidic) dissociation constants and standard oxidation-reduction potentials of the Co3+/Co2+ and Co2+/Co1+ couples of isolated aquo-cba-HBI were measured. Comparison of the data to those established for 5,6-dimethylbenzimidazolylcobamide (cobalamin) showed that the 5-hydroxybenzimidazolyl (HBI) nucleotidic base hardly affected the redox potentials. HBI, however, proved to be the weaker ligand, thus favoring "base-off" formation. The implications for the functioning of cba-HBI in biochemical methyl group transfer reactions are discussed.

Characterization and determination of the redox properties of the 2[4Fe-4S] ferredoxin from Methanosarcina barkeri strain MS

Ferredoxin was purified from methanol-grown Methanosarcina barkeri strain MS. It was isolated as a dimer with a subunit molecular weight of 6,200. The protein contained 7.4 mol iron and 7.2 mol acid-labile sulfur per monomer. In the reduced state the ferredoxin exhibited an EPR spectrum characteristic of two spin-coupled [4Fe-4S]1+ clusters. The EM of the [4Fe-4S]2+:1+ couple was -322 mV +/- 3 mV vs. NHE at 21 degrees C and pH 7.0. The midpoint potential was temperature but not pH dependent. At the physiological temperature of 37 degrees C the Em was -340 mV.

Purification and characterization of coenzyme F390 synthetase from Methanobacterium thermoautotrophicum (strain delta H)

Coenzyme F390 synthetase catalyzes the formation of 8-hydroxyadenylylated-coenzyme F420 (coenzyme F390-A) from coenzyme F420 and ATP in some methanogenic Archaea. The presence of coenzyme F390 was found when these organisms were exposed to oxygen. To get more insight into the defined function of coenzyme F390, the coenzyme F390 synthetase from Methanobacterium thermoautrophicum was purified from a cell-free extract and its catalytic properties were determined. The synthetase was purified 150-fold to a specific activity of 0.45 mumol.min-1.mg protein-1. The enzyme consisted of one polypeptide of approximately 51 kDa. The isolated enzyme showed a tendency to aggregate into dimers and tetramers upon concentration. Co-elution during purification of GTP-dependent coenzyme F390 synthetase activity suggested that the synthetase is also capable of 8-hydroxyguanylylated-coenzyme F420 (coenzyme F390-G) formation. Initial-velocity measurements of the two-substrate reaction showed that the enzyme kinetics for the coenzyme F390 synthetase reaction proceeded by a ternary-complex mechanism. The coenzyme F390 synthetase displayed a Km for coenzyme F420 of 39 microM and a Km for ATP of 1.7 mM. In contrast to the enzyme in the cell-free extract, the isolated enzyme was active under aerobic and anaerobic conditions. Treatment with air was not required to obtain the enzyme in an active form. However, 1,5-dihydro-coenzyme F420 (coenzyme F420H2) appeared to be a potent competitive inhibitor (Ki 3 microM) with respect to coenzyme F420. The latter findings may explain why the enzyme could only be detected in crude extracts that had been exposed to air, i.e. treatment with air causes the oxidation of reduced coenzyme F420 present in anaerobic extracts. The results of this study are discussed in view of the proposed role for coenzyme F390 in methanogenic metabolism.

Purification and characterization of inorganic pyrophosphatase from Methanobacterium thermoautotrophicum (strain delta H)

Inorganic pyrophosphatase (EC 3.6.1.1.) has been isolated from the archaebacterium Methanobacterium thermoautotrophicum (strain delta H). The enzyme was purified 850-fold in three steps to electrophoretic homogeneity. The soluble pyrophosphatase consists of four identical subunits: the molecular mass of the native enzyme estimated by gel filtration was approx. 100 kDa and denaturing polyacrylamide gel electrophoresis gave a single band of 25 kDa. The enzyme also may occur as an active dimer formed by dissociation of the tetramer. The pyrophosphate showed an optimal activity at 70 degrees C and a pH of 7.7 (at 60 degrees C) and was not influenced by dithiothreitol, sodium dithionite or potassium chloride. The enzyme was very specific for pyrophosphate (PPi) and Mg2+. Magnesium could be partially replaced by Co2+ (15%). The reaction was inhibited for 60% by 1 mM Mn2+ in the presence of 24 mM Mg2+. In addition, the enzyme was inhibited by potassium fluoride (50% at 0.9 mM). Kinetic analysis revealed positive co-operativity for both Mg2+ and PPi with Hill coefficients of 3.3 and 2.0, respectively. Under the experimental conditions at which the enzyme was present as its dimer, the apparent Km of PPi and magnesium were determined and were approx. 0.16 mM and 4.9 mM, respectively; Vmax was estimated at about 570 U/mg.

Involvement of an activation protein in the methanol:2-mercaptoethanesulfonic acid methyltransferase reaction in Methanosarcina barkeri

Methanol:5-hydroxybenzimidazolylcobamide methyltransferase (MT1) is the first of two enzymes required for transfer of the methyl group of methanol to 2-mercaptoethanesulfonic acid in Methanosarcina barkeri. MT1 binds the methyl group of methanol to its corrinoid prosthetic group only when the central cobalt atom of the corrinoid is present in the highly reduced Co(I) state. However, upon manipulation of MT1 and even during catalysis, the enzyme becomes inactivated as the result of Co(I) oxidation. Reactivation requires H2, hydrogenase, and ATP. Ferredoxin stimulated the apparent reaction rate of methyl group transfer. Here we report that one more protein fraction was found essential for the overall reaction and, more specifically, for formation of the methylated MT1 intermediate. The more of the protein that was present, the shorter the delay of the start of methyl group transfer. The maximum velocity of methyl transfer was not substantially affected by these varying amounts of protein. This demonstrated that the protein was involved in the activation of MT1. Therefore, it was called methyltransferase activation protein.

5-Formyl-5,6,7,8-tetrahydromethanopterin is the intermediate in the process of methanogenesis in Methanosarcina barkeri

Formylmethanofuran:tetrahydromethanopterin (H4MPT) formyltransferase and 5,10-methenyl-H4MPT cyclohydrolase purified from Methanosarcina barkeri catalyze a formyl group transfer and the hydrolysis of the methenyl function, respectively. The results from UV spectroscopy and HPLC analyses, and comparison with results obtained with the enzymes isolated from Methanobacterium thermoautotrophicum showed 5-formyl-H4MPT to be the product of the formyltransferase and cyclohydrolase reactions in M. barkeri. The findings disagree with an earlier report in which 10-formyl-H4MPT was identified as the product of the cyclohydrolase in the latter organism. In addition, it was observed that 10-formyl-H4MPT, which is non-enzymically formed from 5,10-methenyl-H4MPT at alkaline pH, becomes rapidly converted into the 5-formyl derivative. The latter finding explains why the nature of the formyl species previously had been improperly assigned.

Isolation of a 5-hydroxybenzimidazolyl cobamide-containing enzyme involved in the methyltetrahydromethanopterin: coenzyme M methyltransferase reaction in Methanobacterium thermoautotrophicum

Formaldehyde conversion into methyl-coenzyme M involves (a) reaction of the substrate with 5,6,7,8-tetrahydromethanopterin (H4MPT) giving 5,10-methylene-H4MPT, followed by its reduction to 5-methyl-H4MPT and (b) transfer of the methyl group from the latter compound to coenzyme M. The reactions were studied in a resolved system from Methanobacterium thermoautotrophicum strain delta H. The first part (a) of the reactions was catalyzed by the 55% ammonium sulfate supernatant of cell-free extracts. The methyltransferase step (b) was dependent on an oxygen-sensitive enzyme, called methyltransferase a (MTa). Isolation of MTa was achieved by gel filtration on Sephacryl S-400. MTa was a high-molecular-weight complex of at least 2000 kDa and between 900 to 1500 kDa when purified in the absence and presence of the detergent CHAPS, respectively. The enzyme consisted of 100 kDa units composed of three subunits in an alpha beta gamma configuration with apparent molecular masses of 35, 33 and 31 kDa, respectively. The corrinoid, 5-hydroxybenzymidazolyl cobamide (B12HBI, Factor III) copurified with MTa and the latter contained 2 nmol B12HBI per mg protein. B12HBI present in MTa could be methylated under the appropriate conditions by 5-methyl-H4MPT. These findings suggest that the corrinoid is a prosthetic group of MTa. MTa may be homologous to the corrinoid membrane protein purified before from M. thermoautotrophicum strain Marburg (Schulz, H., Albracht, S.P.J., Coremans, J.M.C.C. and Fuchs, G. (1988) Eur. J. Biochem. 171, 589-597).

Purification and properties of 5,10-methylenetetrahydromethanopterin dehydrogenase and 5,10-methylenetetrahydromethanopterin reductase, two coenzyme F420-dependent enzymes, from Methanosarcina barkeri

5,10-Methylenetetrahydromethanopterin dehydrogenase and 5,10-methylenetetrahydromethanopterin reductase have been purified to homogeneity by a factor of 86 and 68, respectively, from methanol-grown Methanosarcina barkeri cells. The dehydrogenase was isolated as a hexamer of a single 35 kDa subunit, whereas the reductase was composed of four identical 38 kDa subunits. The purified oxygen-stable enzymes catalyzed the oxidation of 5,10-methylenetetrahydromethanopterin and methyltetrahydromethanopterin with Vmax values of 3000 and 200 mumol min-1 mg-1, respectively. The methanogenic electron carrier coenzyme F420 was a specific electron acceptor for both enzymes. Steady state kinetics for the two enzymes were in agreement with ternary complex (sequential) mechanisms. Methylene reductase and methylene dehydrogenase are proposed to function in the methanol oxidation step to CO2.

Hydrolysis and reduction of factor 390 by cell extracts of Methanobacterium thermoautotrophicum (strain delta H)

Cell extracts of Methanobacterium thermoautotrophicum (strain delta H) were found to perform a hydrogen-dependent reduction of factor 390 (F390), the 8-adenylyl derivative of coenzyme F420. Upon resolution of cell extracts, F390-reducing activity copurified with the coenzyme F420-dependent hydrogenase. This indicates that F390 serves as a substrate of that enzyme. Activity towards F390 was approximately 40-fold lower than that towards coenzyme F420 (0.12 and 5.2 mumol.min-1.mg of protein-1, respectively). In addition, cell extracts catalyzed the hydrolysis of F390 to AMP and coenzyme F420. This hydrolysis required the presence of thiols (6 mM) and much ionic strength (1 M KCl) and was reversibly inhibited by oxygen. The reaction proceeded optimally at pH 8.2 and was Mn dependent. Conditions for F390 hydrolysis in cell extracts are in many respects opposite to those previously described for F390 synthesis.

Identification of para -Cresol as a Growth Factor for Methanoplanus endosymbiosus

Growth of the methanogenic bacterium Methanoplanus endosymbiosus is dependent on the presence of ruminal fluid. Ruminal fluid could be replaced by the eluate of a rumen-derived anaerobic digester. From the eluate of the digester, a growth-stimulatory component was purified and identified as p -cresol. Authentic p -cresol supported a half-maximal growth rate of the organism at 50 nM concentration.

Purification and characterization of coenzyme F420-dependent 5,10-methylenetetrahydromethanopterin dehydrogenase from Methanobacterium thermoautotrophicum strain delta H

5,10-Methylenetetrahydromethanopterin dehydrogenase from Methanobacterium thermoautotrophicum strain delta H was purified to homogeneity with nearly complete recovery. The aerobically stable monofunctional enzyme catalyzed the reversible oxidation of 5,10-methylene-5,6,7,8-tetrahydromethanopterin to its 5,10-methenyl derivative. For the reaction a midpoint potential E'0 = - 362 mV was calculated at 60 degrees C. The methanogenic electron carrier coenzyme F420 was strictly required as the co-substrate. The dehydrogenase (Mr 216,000) was purified as an apparent hexamer of six identical 36 kDa subunits. Oxidation of 5,10-methylenetetrahydromethanopterin coupled to coenzyme F420 reduction catalyzed by the dehydrogenase with a turnover number of 2400 S-1 proceeded via a ternary complex mechanism. High concentrations of monovalent cations markedly stimulated the reaction.